Momentum control devices, most notably control moment gyroscopes and reaction wheels, are commonly deployed within attitude control systems aboard vehicles, such as spacecraft. A generalized momentum control device includes a rotor assembly rotatably mounted within a rotor assembly housing. The rotor assembly includes an inertial element, typically a rotating mass or an outer rim, which is fixedly coupled to a rotor shaft. The first end of the rotor shaft (often referred to as the “fixed end” of the rotor shaft) is mounted within a first bore provided within the rotor assembly housing such that the first end forms a rotating portion of the rotor assembly, but is otherwise confined, relative to the rotor assembly housing. The second end of the rotor shaft (the “floating end” of the rotor shaft) is suspended within a second bore provided in the rotor assembly such that the second end is able to move axially and radially within certain limits, as well as rotate, relative to the rotor assembly housing. A bearing (e.g., a duplex-pair ball bearing) is disposed over each shaft end to facilitate rotation of the rotor assembly. During operation of a momentum control device, a spin motor causes the rotor assembly to rotate about a spin axis. If the momentum control device assumes the form of a reaction wheel, the rotor assembly housing may be directly mounted to a vehicle such as spacecraft. If the momentum control device assumes the form of a control moment gyroscope (“CMG”), the rotor assembly housing may be rotatably disposed within an outer stator housing (e.g., a basering structure), which may be mounted to a vehicle.
With respect to a Reaction Wheel Assembly (RWA), when adjustment of the attitude of the host vehicle is required, a spin motor drives the rotational speed and/or direction of the rotor shaft, resulting in a momentum change. The momentum change and resulting output torque from the RWA is transferred to the host vehicle, effectuating an attitude adjustment. A single RWA is typically combined with two or more additional RWAs to produce a Reaction Wheel Assembly Array (RWAA). An RWAA containing at least three RWAs can perform highly controlled attitude adjustments about three orthogonal axes for the host vehicle.
During operation of the RWA, the spin motor drives rotation of the rotor shaft about the spin axis over a wide range of rotational speeds (for example, from −6000 to +6000 revolutions per minute (RPM)), thus establishing the momentum. The RWA motor hall-effect sensors form a crude, low resolution encoder typically relied upon to measure the (i) rotational frequency of the rotor shaft, and (ii) sign of the rotor shaft velocity. Current RWA designs generally implement a low resolution tachometer using these measurements to minimize cost and improve reliability; however, such tachometers generally do not meet preferred performance specifications when rotational speed is near zero due to the limited resolution of the digital encoder.
To compensate for the inadequate near-zero RPM performance of conventional tachometers, RWAs typically use a use-case technique referred to as “speed biasing” to avoid going through zero RPM during minor momentum exchanges. A speed biased RWA has its “zero” momentum origin arbitrarily set at a predetermined rotational velocity away from true zero RPMs. This is accomplished by dumping vehicle momentum until the RWAA zero momentum state is at a low RWA rotor velocity greater than zero. This allows the vehicle to navigate without reversing the RWA rotor spin direction often. However, by avoiding true zero RPM, speed biasing subjects the RWA to constant elevated spin speed, each of which accelerates degradation of components and reduces the overall lifespan of the RWA. This is a drawback, as many RWA applications require extended lifespans.
Accordingly, a rotor assembly that enables near-zero revolutions per minute (RPM) sensing for deployment within a momentum control device, and method for making same is desirable. The desirable rotor assembly utilizes a magnet coupled to the rotor shaft and a sensor element to detect magnetic flux from the magnet and derive reliable near zero RPM data therefrom.